This invention concerns electrical resistors, and more particularly, film resistors which are amenable to high speed laser trimming as practiced in the microcircuits industry. The term laser trimming relates to the technique of adjusting the electrical resistance of a film resistor by laser removal of resistor material until desired resistance value is achieved. The removal is accomplished by thermal vaporization of the resistor material at the interface of a laser beam and the resistor surface. See, for example, the E. I. du Pont de Nemours & Co. publication No. A-99397 (7/74), "Laser Trimming Techniques For Thick Film Resistors".
During laser trimming, products of vaporization condense along the border of the laser-scribed path, forming an adherent residue. The effect of the residue on trimmed resistance measurement stability has generally not created a problem unless the resistance of the trimmed resistor exceeds 10.sup.6 ohms i.e., in the case of megohm resistors. As to the latter, the residue effectively forms an electrically conductive layer upon the resistor surface which interferes with reliable resistance measurement of the resistor during and after the laser trimming process. The result is the prevention of accurate and reliable adjustment of megohm resistors when using conventional laser trimming procedures.